Heart-rate variability (HRV), a measure of fluctuation around the mean heart rate, reflects the sympathetic and parasympathetic balance of the autonomic nervous system, and is an excellent technique to study cardiovascular tone in patients with neurological injuries. The purpose of this study was to determine whether abnormal HRV is present in patients with traumatic brain injury (TBI) during the post-acute recovery phase. Using a prospective, case/control design, we performed 24-h ambulatory ECG monitoring in seven TBI patients and in seven controls (C). There was a significant difference in root mean squared successive difference of RR intervals (C 40.4 +/- 10.3, TBI 23.3 +/- 16.5, p = 0.04) between TBI and C. Four patients with TBI (compared to one control) had abnormal standard deviation of the RR interval. When these four patients were compared to their matched controls, significant differences were found in frequency domain measure (In total power: TBI 4.4 +/- 0.9 ms2, C 7.1 +/- 1.4 ms2, In low frequency: TBI 3.3 +/- 1.1 ms2, C 6.4 +/- 1.4 ms2; In high frequency TBI 2.0 +/- 1.0 ms2, C 4.8 +/- 1.3 ms2, all p < 0.05). Thus, abnormalities in both time and frequency domains of HRV are present in TBI during the post-acute recovery phase.